The present invention relates to electric meters and, more particularly, to apparatus and method for accumulating demand in an electronic demand register of an electric meter.
Conventional electric meters employ an aluminum disk driven as a rotor of a small induction motor by an electric field at a speed which is proportional to the electric power being consumed by a load. Geared dials, or cyclometer discs, integrate the disk motion to indicate the total energy consumed, conventionally measured in kilowatt hours (one kilowatt hour equals one thousand watts of power consumption for one hour).
Maximum demand is conventionally defined as the maximum amount of power consumed in any one contiguous time interval during the time period of interest; that is, the maximum amount of power consumed in any one of the intervals of, for example, 15 minutes, 30 minutes or one hour over a time period. The time period may, for example, coincide with the time between readings of the meter. From the standpoint of the utility providing the power, the capital investment required for power generation equipment is determined by the amount of equipment required to supply the peak, or maximum, amount of power which may be demanded by all of its users. It has been widely observed that system-wide power usage varies widely with time of day and with season of year. In order to meet the average, or base level, of power usage, a utility can provide a large, efficient, base load generator which may be, for example, a hydroelectric or steam turbine-generator. Such base load generators are typically maintained in continuous service for many years without being either shut down or totally unloaded. In such continuous service, base load generators provide high thermodynamic efficiency. To supply the peak loads, a utility is required to bring on line generating equipment having lower thermodynamic efficiency. Thus, not only does the capital cost for power generation increase when a high peak load to base load ratio exists, but also, the operating costs for such peak-load generating equipment is higher than that for base-load generating equipment.
The system load imposed on a utility is, in the end, determined by the energy consumers. If the energy consumers can be convinced to reduce the peaks in their individual energy consumption, the peaks in system load will also be attenuated. Demand metering has been introduced to give the energy consumers an economic incentive to reduce the peaks in their energy consumption. This is accomplished by keeping track of the amount of energy used in each of a series of demand intervals and storing the maximum value of demand measured in any one of the demand intervals. The measured maximum demand is then used to modify the consumer's billing so that the bills are lower if the maximum demand is lower and are higher if the maximum demand is higher. This method of modifying consumer billing on the basis of the maximum demand has been accepted by many of the governmental utility commissions which oversee billing practices.
One type of electric meter containing a mechanical demand register is disclosed in U.S. Pat. No. 3,586,974. The mechanical demand register records the power usage in each of a contiguous series of demand intevals. A demand interval may be, for example, a predetermined fraction of an hour, the demand register stores the maximum value occurring during any one of the demand intervals for reading at the end of a predetermined period of time. The predetermined period of time may be, for example, the time between meter readings, or a period of time corresponding to the billing period of the utility providing the power. A clockwork mechanism restarts the demand register at the end of each demand interval so that, when the stored value is read, the stored value represents the highest value of power usage occurring during any one of the demand intervals since the demand register was last reset.
Demand registers of the mechanical type, such as disclosed in the above U.S. Patent, have limited flexibility. Once their design is completed for a particular meter physical configuration, the design is not transferrable to a meter having a different physical configuration. Also, the demand-measurement functions cannot be redefined without major mechanical redesign.
Greater flexibility in demand metering may be obtainable using electronic acquisition, integration and processing of power usage. An electronic processor such as, for example, a microprocessor, may be employed to manage the acquisition, storage, processing and display of the usage and demand data. U.S. Pat. Nos. 4,179,654; 4,197,582; 4,229,795; 4,283,772; 4,301,508; 4,361,872 and 4,368,519, among others, illustrate the flexibility that electronic processing brings to the power and energy usage measurement. Each of these electronic measurement devices includes means for producing an electronic signal having a characteristic such as, for example, a frequency or a pulse repetition rate, which is related to the rate of power usage. The electronic processor is substituted for the mechanical demand register of the prior art to keep track of the demand during defined periods of time.
It has been observed that the maximum demand recorded by such a demand register may vary substantially depending on the size of the demand interval. An excessively lengthy demand interval, for example, tends to average out the peaks in maximum demand to the point that demand metering becomes less meaningful. A too short demand interval can make the demand register almost solely responsive to transients such as occasioned, for example, by motor starting loads.
The recorded maximum demand can also be affected by the accidental positioning of demand peaks and valleys either entirely within a demand interval or bridging two demand intervals. That is, if a single large peak should occur entirely within a demand interval, such a single large peak may raise the total power usage in the demand interval to a value which becomes recorded as the maximum demand and therefore affects the consumer billing. Conversely, if such a single large peak should fortuitously bridge the end of one demand interval and the beginning of the next demand interval, its effect may be split between the two demand intervals in a manner which may so dilute the effect of the peak that neither demand interval may accumulate a usage which exceeds the previously recorded maximum demand. In this latter case, the consumer's billing is not affected in the same way by the peak.